1. Field of the Invention
The present invention relates to a portable information device, in particular, a portable information device on which an RFID (radio frequency identification) for making communication with an external apparatus through magnetic field signals is mounted.
2. Description of the Related Art
An example wherein an RFID is mounted, which is one of automatic recognition techniques for communicating ID (identification) data and various data to an external apparatus, using a magnetic field having a given frequency and radiated from the external apparatus as a signal carrier wave, is the so-called non-contact IC card, such as an IC telephone card or an electronic ticket. In the case that an RFID is mounted on a portable information device and is used, it is indispensable to ensure communication distance. It is required to exclude the effect of obstacles on a magnetic circuit for this.
It is known for this purpose to arrange a flexible magnetic material having a permeability of 100 or more about an excitation frequency of 100 KHz or less proximately to the rear of a loop antenna as a measure for being not affected by any magnetic circuit obstacle with ease (see, for example, Document 1: JP-A No. 9-284038).
There is also known a product wherein a flexible magnetic material is arranged proximately to a loop antenna in the same manner but the value of the relative permeability thereof is not specified (see, for example, Document 2: JP-A No. 2000-276565 and Document 3: JP-A No. 2001-68916).
There is also known a circuit for matching the input-impedance of an LSI with the output-impedance at an intermediate tap point of a resonance circuit composed of an antenna coil and capacitors connected to both ends thereof (the intermediate tap point may be positioned outside the resonance circuit dependently on the impedance) (See Document 4: JP-A No. 2000-278172.)
In order to make communication distance long, to make the resonance frequency of a receiver circuit of an RFID consistent with the frequency of a signal carrier wave from an external apparatus has been performed. For example, in RFIDs of a non contact type IC card, it is known to trim the use number of rows of fine condensers for resonance, which are made of printed patterns, in each of the RFIDs (See Patent document 5: JP-A No. 11-353440).